1. Field of the invention
The present invention relates to a data processor for performing a plurality of operations in parallel, and more particularly to a data processor for performing a plurality of operations in parallel at a high efficiency by executing a so-called VLIW (Very Long Instruction Word) type instruction which specifies a plurality of operations by one instruction.
2. Description of Related Art
As a data processor for performing operations at a high speed utilizing parallelism of instruction levels, data processors adopting superscalar type or VLIW type parallel operation techniques have been already proposed. So-called superscalar is a parallel operation technique in which parallelism of instruction levels is detected by hardware from an instruction stream and a plurality of instructions are then executed in parallel.
On the other hand, the VLIW technique is a parallel operation technique in which one instruction consists of a plurality of parallel executable operations which are detected and encoded by a compiler a t compiling time. A plurality of operations which are specified by this long instruction word are executed in parallel. These types of conventional data processors are described in detail in "Instruction-Level Parallelism", B. R. Rau and J. A. Fisher, The Journal of Supercomputing, Vol. 7, No. 1/2, 1993, for example.
The conventional superscalar type data processor is advantageous in being capable of executing instructions that were generated in the past without translating. However, since the number of instructions which are executable in parallel is not constant, it is necessary to specify instructions which are executable in parallel or to align instructions which are to be inputted to an instruction decoder in accordance with the number of instructions which are executable in parallel. This imposes a large load on the decoder hardware.
Further, since the conventional VLIW type data processor guarantees that instructions have the same length and all of a plurality of operations which are described within one instruction can be executed in parallel, the problem of an increased load on the decoder hardware is not generated unlike in the conventional superscalar type data processor as mentioned above. However, since the conventional VLIW type data processor consumes one instruction even when there is no operations which can be executed in parallel, a number of operation fields specifying null operations (No Operation: NOP) are generated, whereby the amount of instruction code becomes very big. In addition, in the conventional VLIW type data processor, types of operations which can be specified by respective operation fields are limited to simplify the instruction decoder and the operation mechanism. Because of the standardized method of specifying operation, flexibility of assigning operations within one instruction is small, and therefore the instruction code efficiency is poor.